Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


I examined variability in the shell form of 25 Littorina obtusata populations from Massachusetts to northern Maine. I chose this scale because the history of one of L. obtusata's principal predators, the crab Carcinus maenas, has changed dramatically in the past 100 years. Before 1900, Carcinus did not occur north of Cape Cod, Massachusetts, but by 1950 it had reached Canadian border. Moreover, in summer water temperatures during average 6-8&\sp\circ&C colder at northern locations. Shell thickness and mass increased and body mass decreased with increasing latitude. to test whether these patterns reflect plasticity in response to predators, snails from two northern and two southern populations were raised in the laboratory with and without Carcinus effluent. Snails raised with Carcinus produced thicker shells than conspecifics raised without Carcinus. This response was accompanied by reduced body size and body growth. Another experiment examined whether geographic differences in water temperature induce changes in shell form. Snails reciprocally transplanted between a northern (Maine) and southern (Massachusetts) exhibited substantial plasticity in shell form. Southern snails transplanted north produced significantly thinner, lighter, shells than snails raised at their native shore, while northern snails transplanted south produced thicker, heavier shells than snails raised at the native shore. In addition, snails producing thicker, heavier shells exhibited reduced body mass and growth. Although patterns in final phenotypes exhibited cogradient variation, growth in both shell thickness and mass exhibited countergradient gradient variation. Most examples of countergradient variation are associated with temperature differences suggesting that differences in water temperature are responsible for this pattern. A third experiment involved reciprocally transplanting snails between the same two sites (temperature effect) and raising them with and without Carcinus effluent. In general, Carcinus effluent and warmer water temperatures induced thicker, heavier shells and reduced body mass and growth. Overall, it appears that predator effluent and water temperature have similar effects on shell form. My work on phenotypic plasticity encourages a more pluralistic view of phenotypic variation. Moreover, my results suggest that phenotypic plasticity is a ubiquitous strategy in adapting to different environments and that its evolution may be driven by life history trade-offs.



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